Children going back to school in Pennsylvania could be walking into classrooms where as many as one out of every five classmates don’t have all the vaccines required by the state.

While many parents believe that disease outbreaks in school are rare because of vaccines, there were a record number of measles cases in the United States in 2014, according to the U.S. Centers for Disease Control and Prevention (CDC). The 668 cases in 27 states was the highest number since measles were considered eliminated in 2000.

And in the 2013-2014 school year, Pennsylvania had one of the worst vaccination rates in the country for the measles, mumps and rubella (MMR) vaccine, according to the CDC.

State Department of Health data analyzed by PublicSource also show that, in the past eight years, measles vaccination rates for kindergarteners in most Pennsylvania counties never reached a key threshold — the point at which disease is unlikely to spread.

There was no clear trend between the state’s urban and rural areas for measles vaccination rates. They were bad in both.

Up-to-date vaccinations are the best way to protect children from “outbreaks that can cause unnecessary illnesses and deaths,” according to the federal agency’s website.

“[A]ll it takes is one child with an infection in a school to cause significant spread to the larger school community,” Dr. Sue Kressly, a pediatrician and the president of the Pennsylvania chapter of the American Academy of Pediatrics, wrote in an email.

Criticism for the state’s low vaccination rates is often directed at families who claim religious and philosophical exemptions.

In reality, most of the children without all of the required vaccines fall under a state regulation that allows them to attend school for up to eight months as they try to meet the state’s vaccination requirements.

Over the past eight years, the number of students provisionally allowed to attend school has routinely exceeded 10 percent in many Pennsylvania counties and is sometimes more than 20 percent of a county’s kindergarteners or 7th graders, according to the PublicSource analysis.

When do children receive the measles vaccine?

The measles, mumps and rubella vaccine is given in two shots.

First shot: 12 to 15 months old

Second shot: 4 to 6 years old

Source: U.S. Centers for Disease Control and Prevention

For Pittsburgh Public Schools, the deadline for students to be vaccinated before they’re no longer admitted to school would be around May 1.

“That’s exactly the time period when you think about measles… they typically peak in the winter, in the first term,” said Wilbert van Panhuis, an epidemiologist at the University of Pittsburgh Graduate School of Public Health. “It’s counterproductive because, after eight months, the school year is almost over.”

Pennsylvania is one of 20 states that allows parents to claim a religious or philosophical exemption.

Those exemptions allow for a parent to not vaccinate “on the basis of a strong moral or ethical conviction similar to a religious belief.”

They can apply to people whose religion conflicts with vaccinations and others who believe a widely discredited study that said vaccines can cause autism spectrum disorder.

In tight-knit religious communities, there’s the potential for an outbreak. The largest measles outbreak last year (383 cases) occurred primarily among the Amish northeast of Columbus, Ohio. Pennsylvania is the other state with a large Amish population.

Over the past four years, non-medical exemptions were taken by only 2 percent of Pennsylvania kindergarteners and 7th graders. Those are the grade levels at which vaccines must be reported to the federal government.

Kressly doesn’t accept patients who don’t believe in vaccinations.

“Vaccines are not a philosophy,” she said. The very existence of a philosophical exemption allowed by the state, Kressly said, reinforces some people’s doubts.

State Reps. Becky Corbin, R-Chester, and Michael Schlossberg, D-Lehigh, introduced a bill this year that would eliminate the philosophical exemption, but it hasn’t moved out of the House health committee. There is a similar Senate bill.

Provisional enrollment

Pennsylvania has had a provisional enrollment period — the eight-month vaccination grace period — since 1988.

There are any number of possible reasons the provisional enrollment rate is so high in Pennsylvania.

Kressly said she thinks some people are hesitant about vaccines; some families don’t feel the urgency; and other families mistakenly think their child is up-to-date on their vaccines.

She encouraged pediatricians to consistently review a child’s vaccination records.

“We, as clinicians, need to use every opportunity to look at vaccines at every point of contact,” she said.

However, many states don’t have eight-month provisional periods like Pennsylvania. For example, some states, including Texas, require all immunizations to be administered within 30 days of the start of school.

Children in the provisional period are on their way to getting all the required vaccines, which is very different than being exempt.

Pennsylvania Secretary of Health Karen Murphy said the large number of students provisionally enrolled was a greater risk to public health and people who cannot be immunized for medical reasons than the number of students with non-medical exemptions.

The Department of Health is narrowing in on the provisional period. Murphy told PublicSource the department is considering eliminating it as early as the 2016-2017 school year.

Pennsylvania also has a medical exemption.

About one half of one percent of Pennsylvania students have claimed a medical exemption from the vaccine requirements; it requires a doctor’s note. A life-threatening allergy or other complications are medical reasons to forgo certain vaccines.

Herd immunity

Last winter’s highly publicized measles outbreak at Disneyland in California spread to 147 people, seven other states, Mexico and Canada. Of the 110 California patients, 28 were “intentionally unvaccinated because of personal beliefs,” according to the CDC.

About 92 percent of California kindergarteners have received the MMR vaccine in each of the past two years, which is roughly 6 percent higher than Pennsylvania’s rate.

Measles can cause a high fever and rash roughly 10 to 14 days after exposure.

Because measles is extremely contagious — one case can cause more than 10 other cases — roughly 95 percent of the population needs to be immunized in order to prevent the disease from spreading.

It’s a concept called herd immunity.

Herd mentality of immunizations
When there are enough people immunized in a community, the spread of a disease can be contained to only a few individuals. (Credit: National Institute of Allergy and Infectious Diseases)

However, the 95-percent figure assumes that this immunity is spread evenly over a population, which doesn’t exist perfectly in reality. There are concentrations of unvaccinated people, as was the case with the Disneyland outbreak.

“These are the risk areas of diseases,” van Panhuis said. “The more they are clustered, the more the disease can find its way. If these clusters are isolated then it’s hard for the virus to find someone to infect.”

For kindergarteners, only five Pennsylvania counties reached that key 95-percent immunization rate for MMR in at least half of the past eight years. The overwhelming majority — 50 of 67 counties — never reached 95-percent MMR immunization in the past eight years.

But missing that herd immunity target doesn’t guarantee an outbreak.

“It’s possible [to] have an 80- or 85-percent immunization rate and not have an epidemic,” van Panhuis said. “That’s just biology. There’s always this uncertainty.”

It matters where an infected person lives, where they travel and with whom they come in contact. One model that simulates some of those variables is the FRED measles simulation program created by the University of Pittsburgh Graduate School of Public Health.

With options for 13 Pennsylvania cities, it simulates a measles outbreak with a 95-percent immunization rate compared to an 80-percent immunization rate.

Cities such as Philadelphia and Pittsburgh fill with red dots representing infected individuals in the 80-percent immunization scenario, while there are very few cases with the higher vaccination rate.

Poor data quality

The state immunization database, maintained by the Pennsylvania Department of Health, contains several inconsistencies. Numbers that are clearly inaccurate are not corrected, and the math doesn’t make sense.

For instance, in 2011-2012, the department reported that little Cameron County managed to fit 470 kindergarteners in its elementary school, up from 41 students the previous year. In a county that typically has about 40 to 60 kindergarteners each year, this was clearly a mistake.

The school district confirmed they actually had 40 kindergarteners, which significantly improved the county’s incorrectly reported 8-percent vaccination rate to 97.5 percent.

This problem is not new. The department first discovered problems in its data in December, which NBC Philadelphia reported on earlier this year.

Department officials say they have instituted a number of changes that they expect should make the data more reliable for the 2015-16 school year.

The department plans to check the data and verify outliers, according to a statement. They said they’ve created a training guide to help schools enter more accurate information.

The Jewish Healthcare Foundation has contributed funding to PublicSource’s healthcare reporting.

Reach Eric Holmberg at 412-315-0266 or at Follow him on Twitter @holmberges.

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Eric Holmberg was a reporter for PublicSource between 2014 and 2016.